Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object, preferably without leaving human-apparent evidence of alteration.
Digital watermarking may be used to modify media content to embed a machine-readable code into the media content. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process.
There are many processes by which media can be processed to encode a digital watermark. Some techniques employ very subtle printing, e.g., of fine lines or dots, which has the effect slightly tinting the media (e.g., a white media can be given a lightish-green cast). To the human observer the tinting appears uniform. Computer analyses of scan data from the media, however, reveals slight localized changes, permitting a multi-bit watermark payload to be discerned. Such printing can be by ink jet, dry offset, wet offset, xerography, etc. Other techniques involve varying the intensity or luminance of pixel colors. Of course there are many other digital watermarking techniques.
Digital watermarking systems typically have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern by altering data samples of the media content. The reading component analyzes content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reading component extracts this information from the detected watermark. U.S. patent application Ser. No. 09/503,881, filed Feb. 14, 2000, discloses various encoding and decoding techniques. U.S. Pat. Nos. 5,862,260 and 6,122,403 disclose still others. Of course, artisans know many other watermarking techniques.
One digital watermarking application involves watermarking print media such as magazine or newspaper advertisements, brochures, artwork, company logos, graphics, pictures and photographs, etc. In this case, a digital watermark is typically embedded in an image that is printed using a multi-color printing process. For example, the printing may include a cyan (C), magenta (M), yellow (Y) and black (K) process. CMYK ink intensities can be varied to modulate an image's luminance to accommodate a digital watermark signal. The individual CMYK values at a given image area (e.g., a pixel or other image area location) of an unmarked image can be represented as (Co, Mo, Yo, Ko), and the individual values for the watermarked given area (or pixel) are (Cm, Mm, Ym, Km). Each color quantity preferably represents an intensity percentage of the corresponding color ink. If the watermark signal requires a luminance change of α at this location, then:(Cm, Mm, Ym, Km)=(Co+αΔC, Mo+αΔM, Yo+αΔY, Ko),   (1)where (ΔC, ΔM, ΔY) forms a scaling vector in CMY coordinates with a normalized projection onto a luminance axis. Different methods can be used to choose the direction of this scaling vector. For example, scaling may occur along a direction between white and an unmarked color, or scaling along the direction between black and the unmarked color. Consider FIGS. 1 and 2 for further illustration.
While the implementation details of watermark encoding schemes vary significantly, a class of watermarking schemes can be modeled as an array of changes to luminance values of a host image. A host image comprises an array of color vectors (e.g., an array of color such as RGB, CMY, CMYK, etc). The image sample may be represented as a vector between white and the pixel color value. To encode a watermark, the luminance of the image sample may be increased or decreased by adding a watermark vector in a variety of directions. This is because many possible watermark vectors have components along the luminance axis. Two useful directions for orienting the watermark vector are along a vector between black and the watermark color, and along a vector between white and the watermark vector. Using the former direction for the watermark vector is shown in FIG. 1. FIG. 1 shows a 3-dimensional color space with cyan (C), magenta (M) and yellow (Y) axes. The bold axis between black and white represents luminance. To make an equivalent luminance change in an image sample of a given color vector (C1, M1, Y1), one may make a corresponding scale to black as shown.
An alternative method of obtaining the same luminance change is to scale the image sample like a vector between white and the sample's color value as shown in FIG. 2. To make an equivalent luminance change, one may make a corresponding scale to white as shown. Of course a linear combination of these two scaling methods (white to color and black to color) can also be used to effect a luminance change.
By using the scale to white method for colors with high yellow content such as yellow, red and green, and scale to black for blue, cyan and magenta, a lower visibility watermark can be encoded with the same detectability.
Once the color vector entries are established, each of the entries is associated with a set of scale factors. The set includes a scale factor for each color component. The specific color components in the implementation depend on the color format of the image. For example, images in an RGB format have scale factors for each of the R, G and B color components. Similarly, images in a CMY format have scale factors for each of the C, M and Y components of each table entry.
Further reference for such scaling techniques may be made to previously mentioned U.S. patent application Ser. No. 09/553,084 (now U.S. Pat. No. 6,590,996).
In watermarking items such as product packaging, brochures, graphics, logos, labels, etc., sometimes an original printed image is rendered using “spot color” inks to reproduce a specific desired color. A spot color is a specific color ink, and in some cases may lie outside the CMYK (or CMY) printing gamut. A spot color can be halftone-screened (or filtered) to achieve various shades of the original spot color. Embedding a digital watermark in a spot color presents a unique set of problems—these are the problems the present invention addresses. One conventional approach often falls short of solving these problems. This conventional approach produces an approximation of the spot color image using only CMY or CMYK inks. A digital watermark signal is embedded through modulating (or changing) various CMYK pixel (or other image area) intensities. Since the original spot color may lie outside the CMYK gamut, this first approach can yield poor results.
The present invention provides various methods to effectively embed a digital watermark signal in a spot color. To watermark an original spot color in a first embodiment, the original spot color is replaced with a combination of spot color and process inks. For example, cyan (C), magenta (M) and yellow (Y) process components are modulated (e.g., intensity or luminance varied) to include a watermark signal. The CMY components are preferably combined with a halftone-screened (or scaled) version of the original spot color (e.g., a percentage of the original spot color intensity) to approximate the original spot color. Preferably, the resulting watermarked spot color approximation closely resembles the original spot color.
In a second embodiment, a spot color is “screened back” to some percentage of its original intensity (e.g., 95%). The watermark signal is added to the screened back spot color as intensity tweaks to modulate the spot color at various pixel locations. As a variation, a spot color is modulated in accordance with a predetermined level along a screening range of the spot color intensity. As another variation, a non-screened-back spot color (e.g., a spot color at 100% intensity) is negatively modulated at various pixel locations to accommodate a watermark signal.
The foregoing and other features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.